What goes on behind the scenes when a VFX studio does one of those ‘real vs. CG’ test shots?

Cat VFX scene

The art of the side-by-side

VFX studios are regularly called upon to match live action characters exactly in CG – it might be for a digital double of an actor, or, in the case of Captain Marvel’s Goose, a photoreal synthetic version of the film’s scene-stealing cat. Many audience members were perhaps surprised to learn that Goose was a digital creation about 70% of the time on screen.

Curious about the moment the visual effects team had to prove it could actually pull off a photoreal CG cat, I asked visual effects supervisor Dominik Zimmerle from Trixter, which along with ILM made a digital Goose for Captain Marvel, what that moment was like when their CG cat went up against the real thing.

Can you tell?

This moment was recently encapsulated in a neat VFX breakdown video where production visual effects supervisor Chris Townsend explained how footage of a real cat was shown in a side-by-side test to the directors, who didn’t know that one of the sides was a CG Goose.

Interestingly, for Trixter, that test was not actually something Marvel requested. “We got a package with all kinds of reference materials that filmed Reggie [the main live-action cat] from every kind of direction,” says Zimmerle. “It had a lot of footage in controlled lighting environments, and so we picked two shots that we thought might be good for Marvel to see what we could achieve with the assets we had.”

One shot was of Reggie going from left to right, and the other was of the cat standing on a box, jumping down from the box, holding for a second, and then coming towards the camera. Trixter roto-mated Goose from these and then went to work on placing its rendered asset into the frames to make the side-by-sides.

To acquire a plate in which to place their CG cat, Trixter artists retouched the original plate with the trained cat. “It was quite simple,” notes Zimmerle, “because it was a static camera, a locked-off camera, and there were no lighting changes to adjust. So, as we would regularly do, we would paint out the cat, and then we had our plate.”

Once Trixter had something that they felt was a close approximation to the real character, Zimmerle concedes that even he was stunning. “When I saw those side-by-sides for the first time myself, I was thinking, ‘Is that a CG cat? Oh my God, that’s a CG cat! Oh cool.'”

They Zimmerle presented this to Townsend. “I think he was really astonished and he showed it to the directors, and they were asking, as far as we heard, ‘Okay, why are we looking at this?’ And then he told them that this side was the CG cat, and they were really impressed by that.”

Eagle-eyed viewers may have noticed that the real Goose in the side-by-sides does not have a collar on, but that the CG cat does. This was because Trixter had look-dev’d the character with a collar as was required for the film, and of course been working hard on solving the groom around it, so the collar stayed in.

A/B’ing a CG cat

Trixter’s process for making the digital Goose in many ways followed a traditional methodology of gathering reference, modeling, texturing, rigging, animating, grooming and rendering the creature. They created gray-shaded versions first, then turntables, fur lookdev – all the steps they would do for any CG model. All the while, however, artists had to constantly look back at the real cat to make sure they were on track.

Plate with a greenscreen stuffie stand-in
Plate with a greenscreen stuffie stand-in.
CG cat model
CG cat model.
CAT VFX final shot
Final shot.

How did they do this? It started with collecting QuickTimes of internet cat footage. “Some of them you cannot download and it’s really just a visual reference, like an inspiration,” says Zimmerle. “Just as if you would bring a cat to the office, which, by the way, we also did.”

Then of course there were references of Reggie and other cat stand-ins provided by production. For these, Zimmerle and his team, including animation supervisor Simone Kraus Townsend, would go frame by frame through the footage.

“For this we use RV,” outlines Zimmerle. “Our tool of choice for reviewing anything movie-related is RV. For the stuff that we want to rotoscope, or bring into Maya, we usually go straight via image frames. So we convert the QuickTimes to still images and bring them in as image frames alongside a match-moved camera or at least an aligned camera that is as close as possible. That gives the animators reference in the viewport so they can match it one-to-one.”

Cat ‘likeness’

The specific cat attributes that needed to be matched one-to-one were many, and, according to Zimmerle, changed over the course of production. “It was very interesting to see how you changed your view on the cat,” he says. “In the beginning when we received all the reference material, we thought we knew all about cat proportions. But then you go, wait, what actually are cat proportions? And then you try to find common things that cats have anatomy-wise or bone structure-wise, say.”

CG cat model
CG cat model.
Trixter's final render
Trixter’s final render.

“Then you look at the fur,” continues Zimmerle, “and you think, oh this looks more fluffy here, more fluffy there, and then you really look at the patterns. For example, how it’s clumping on the chest area, how it’s behaving on the rear area, how the tail is looking, how the belly fur is looking. The entire team gets better and better at looking at Reggie. Basically it’s a little bit of a race – you stay ahead of the results before people see that it’s a CG cat.”

What Zimmerle means here is that often their first beauty passes of the CG cat would be received as widely impressive. But as the production continued, he says Trixter artists got better – and the client got better – at scrutinising the work. “So the first time you show it, it’s like, ‘Oh my god, this looks like a cat.’ And then you start, bit by bit, going into the details, things like the dots on the nose or the stripes on the face being in the same direction. You really analyze this whole cat, bit by bit from front to tail.”

That attention to detail happened in animation, too. Says Zimmerle: “It was about fine-tuning animation of how the left eye turns, or doing quicker head turns, having left-ear movement, or quicker ear movement, more asymmetry in there, the way the cat is looking around, the way a cat is walking – all these fine subtleties of animation that Simone and her team were taking great care of.”

Goose almost takes flight
Goose almost takes flight.
The final shot
The final shot.

One thing the Trixter team looked out for was ensuring their cat likeness did not sway into dog-like behavior. “It very easily can get like a dog,” notes Zimmerle. “For example, dogs can kind of tilt their eyes and the eye sockets a lot, whereas cats cannot. Dogs have a more smooth behaviour in how they move their body, and cats are little bit more staccato, and have more attention span in to one direction, then quickly change to another direction, with a little bit more pose-to-pose.”

The end result is that a majority of the Goose shots in Captain Marvel are CG. Asked whether the task of having to match to a real cat was a daunting one, Zimmerle says his team was simply excited to be given the opportunity. Along the way they were buoyed by early positive results, and comments back from Chris Townsend who, Zimmerle recounts, said to the team, ‘Okay, you seem to get cats.’

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